We have developed a model of legionnaires' pneumonia in guinea pigs and rats. A reproducible population of Legionella pneumophila was obtained in late exponential growth phase and inoculated into the trachea of young animals. Either immunologic or microbiologic evidence of infection was demonstrated in 27 or 28 guinea pigs and 19 of 20 rats that had been inoculated with 10(5) to 10(7) colony-forming units. An acute pneumonia that resembled human legionnaires' disease was produced in both species, and Legionella antigen was closely related to inflammation in the distal air spaces. A fatal illness was produced in guinea pigs, and pneumonia was more extensive than in rats. Extrapulmonary inflammatory lesions, particularly splenic necrosis, were more frequent in guinea pigs than in rats. Each rodent species has potential advantages for testing specific questions and both should be useful for future investigations.